This invention relates in general to integrated circuit packaging techniques and in particular, to a method of packaging an integrated circuit die in a molded plastic package including the formation of a coating of stress relief material on a surface of the integrated circuit die prior to transfer molding the integrated circuit die.
Conventional techniques for packaging integrated circuit ("IC") die in molded plastic packages can cause cracks in the die as a result of stress created by thermal expansion mismatch between the die and the molding compound. In FIG. 1a, for example, a cross-sectional view of a plastic packaged IC 5 is shown in which an IC die 10 is mechanically supported by a die attach pad 12 which is itself supported by and connected to a lead frame having a number of leads such as lead 14. Electrical connection from a bond pad (not shown) on the IC die 10 to the lead 14 is made by a bonding wire 16 which is attached to the IC die 10 at bonding point 20 and attached to the lead 14 at bonding point 18. Similar electrical connections are made by bonding other wires to other bond pads of the die and other leads of the lead frame. The thus connected die and lead frame structure is then encapsulated in a molded plastic package 22 by conventional transfer molding techniques. After injection molding, the plastic is allowed to cool back to room temperature and as shown in FIG. 1b, contraction forces 30, 32, 34 and 36 act against the IC die 10 and lead frame structure.
In order to reduce the stress on the die surface, die coatings with lower modulus of elasticity and/or thermal coefficients of expansion characteristics have been applied to the die surface to act as a buffer or stress relief layer between the molded plastic package and the die surface. Although these methods have been satisfactory for coating relatively small die surface areas, they do not promise such satisfactory results for coating the larger die surface areas that are becoming prevalent as die sizes approach wafer scale integration sizes.
Under current die coating processes, the resulting die coating layer (also referred to herein as "stress relief layer") tends to "mound up" and generally either fails to cover the entire die surface or if it does cover the entire die surface, tends to cause thermally induced shearing forces at the die coating to molded plastic interface that can cut the wires bonded to the die.
The effects of other problems with current die coating processes are also expected to increase as the dimensions of IC die increase. First of all, the combined stress and shearing forces on the die surface caused by the hot plastic mold material contracting as it cools to room temperature, increase exponentially from the center of the die to the outer edges of the die. If the die coating fails to cover the entire surface of the die, it is the outer edges which are being subjected to the largest thermal forces that are left uncovered and consequently, are the areas most likely on the die to experience cracking.
Secondly, as the dimensions of the IC die increase, the height of the mound of die coating material increases accordingly. As the height of the mound of die coating material increases in the center of the die, the molded plastic material over this area correspondingly decreases and as a result, the mechanical strength and integrity of the plastic package over this area diminishes.
For example, FIG. 2 illustrates a layer of stress relief material 26 on the surface 24 of IC die 10. The IC die 10 is otherwise wire bonded to a lead frame and packaged in molded plastic as described in reference to FIG. 1a. In FIG. 2, however, IC die 10 is buffered from the contracting forces 30 of the molded plastic 22 by the layer of stress relief material 26. By selecting a stress relief material with a lower modulus of elasticity and/or thermal coefficient of expansion than the plastic material 22, the contracting forces 33 caused by the stress relief material cooling to room temperature along with the molded plastic 22, places less stress on the die surface 24 than the forces 30 of the molded plastic 22.
The stress relief layer 26 is shown to be shaped like a mound having a height d2. As the height of the stress relief layer 26 increases, the thickness d1 of the plastic material 22 above the surface of the stress relief layer 26 correspondingly decreases. As a result, the mechanical strength and integrity of the plastic package at this point is reduced. To ensure that the entire die surface 24 is covered with stress relief material, however, it is desirable to increase the height of the stress relief layer 26. Thus, because the stress relief material 26 has a tendency to form a mound, a trade-off is required between either not fully covering the die surface, or fully covering the die surface and consequently, weakening the mechanical strength and integrity of the plastic package over the center of the die.